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Catalytic Converter (CAT)

The catalytic converter is a device installed in the exhaust system to significantly reduce the emission levels of hydrocarbons (HC), carbon monoxide (CO), and in the case of most newer catalytic converters, oxides of nitrogen (NOx). CO and HC reduction only catalytic converters are referred to as Two Way CATs. CAT is an abbreviation of catalytic converter. Catalytic converters which reduce all three emissions are called Three-Way CATs. Your vehicle's Underhood Emissions Label will provide you with information regarding the type and requirement of this component, along with the engine class the CAT must be designed for.

Most vehicles are required to be equipped with a catalytic converter. Some older vehicles and heavy duty trucks may be exempt. If you vehicle requires one (or two or three, depending on the model vehicle) it must be present, installed and functioning properly.

Operation: The Catalytic converter is a catalyst, it plays no part on how well your engine runs. The catalytic converter only begins working after exhaust has exited the engine's combustion chambers. When hot exhaust gases are forced through the catalytic converter, they contact the catalyst substrate. This causes a rapid increase in temperature, causing exhaust gases to burn hotter, ultimately reducing emissions.

In the 3-way catalytic converter, once the HC and CO reduction has occurred by extreme heating, a NOx reduction substrate separates the harmful pollutant known as nitric oxides into nitrogen and oxygen.

Catalysts are designed to function for a limited time, and require replacement.

How long a vehicles catalytic converter lasts will depend on how completely the engine burns fuel before presenting it the to the catalytic converter and also on the quality of the CAT. Factory CATs should normally last 7 to 10 years. Catalytic converter failures most often occur on vehicles which have been poorly maintained and which have produced high emissions in more then one category.

Location: The CAT is normally located in the exhaust system before the muffler and after the exhaust manifold.

Which Type Should I Choose?


The catalytic converter exterior is usually made of stainless steel. The catalytic converter looks very similar to a muffler except it contains a catalyst substrate within. Most often you will fiind the catalytic converter located underneath your vehicle towards the center of the exhaust system. On certain 4 and 6 cylinder model engines you will see the CAT directly after the exhaust manifold. On these model vehicles the CAT and exhaust manifold may be one unit, and may include a second or third CAT further down the exhaust system; but always before the muffler.

For the Smog Test: During the smog inspection the smog technician will visually inspect the catalytic converter for presence and proper connection. Should your vehicle fail the smog test the CAT may be suspect. A vehicle's catalytic converter usually becomes plugged after a material breakdown within it normally due high exhaust temperatures and rich fuel. After first inspecting the fuel and ignition systems, an intrusive CAT test and Back Pressure/Vacuum test may be performed. These tests will determine whether the catalytic converter is plugged. The important thing here is this... if in fact the CAT is damaged, it is important to ensure a pre-existing problem is not the culprit.

You do not want to damage a new catalytic converter after performing a replacement. A CAT melt down usually occurs due to high CO (Rich Fuel), over time, and extreme heat. It is recommend to suspect a defective CAT only after a complete emission systems diagnosis has been performed.

Symptoms of a Defective Catalytic Converter:

  • Symptom: Loss of engine power over 15-20 miles per hour.

    Fault: CAT is plugged up and restricting exhaust flow.

  • Symptom: Strong sulfur (egg) smell from good running engine.

    Fault: Catalyst not completing the burning process properly.

  • Symptom: Rattle being heard from CAT during idle and acceleration.

    Fault: Catalytic substrate broken down and possibly plugged.

How to find out if a catalytic converter is defective?

The most accurate way to find out if your vehicle's CAT is working efficiently is by using an exhaust gas analyzer. Unfortunately this tool is fairly expensive and not designed for home use. You must visit a local smog station and have the smog technician inspect the CAT via the stations smog machine (BAR-97 Unit) or stand-alone 5 gas analyzer.

California Catalytic Converter Laws and Regulations

The smog stations says my catalytic converter is not legal in California. Why?

Since the implementation of the new STAR smog check program, smog stations are very careful in ensuring vehicles are equipped with proper/required catalytic converters. The smog check technician will check your vehicle's catalytic converter for an EO number/stamp. If the EO (executive order) number doesn't not match the correct EO number as indicated on the California Air Resources Board website for Allowed Aftermarket Catalytic Converters, your vehicle will fail the visual portion of the smog check as an "emissions tamper".

My vehicle failed the smog check for CAT Wrong Application

If your vehicle has passed smog checks in the past with a catalytic converter which is not passing now, and you are certain the catalytic converter used to be allowed for use on your vehicle, you might have recourse.

Under current regulations a pre-OBD II (pre-2009) converter can pass the smog check as long as it was installed before 1/1/09, and you can provide the smog station proof of the installation date and proof that the CAT was designed specifically for use on your vehicle (including make, model, year, and engine size). The following is a PDF by the California ARB explaining the new California CAT law. Contact the California Smog Check Referee Office at (800) 622-7733 for more information or to make a smog inspection appointment.

If you can not prove your vehicle's catalytic converter is of proper fit and design, or that it was installed before 2009, the following ARB Aftermarket Catalytic Converter Lookup Table will show you which ARB approved CATs are currently allowed for your make, model, year, and engine size of vehicle.